The military subsequently confirmed "a mini submarine" had violated its territorial waters, but was never able to establish the vessel's nationality.

Tengroth said on Thursday that after the April 21st-24th exercises were suspended, the military launched maritime surveillance in the area and divers were sent down to investigate.

"Tracks were found on the seabed (…) but they were nothing that could be linked to these indications," he said.

He also stressed that the phrase "unknown underwater activity" had a broad definition and did not only apply to submarines.

Sweden's centre-left government announced last year a 10.2 billion kronor ($1.25 billion) increase in defence spending for 2016-2020 amid concerns over Russia's military resurgence.

A non-Nato country, Sweden has also stepped up its military capabilities and exercises with the alliance, amid signs of more assertive Russian behaviour in the Baltic region, including Russian planes regularly skirting or violating the national airspace of neighbouring countries.